THYROID-HORMONE EFFECTS ON CONTRACTILITY AND MYOSIN COMPOSITION OF SOLEUS MUSCLE AND SINGLE FIBERS FROM YOUNG AND OLD RATS

1. Young (3-6 months) and old (20-24 months) male Wistar rat soleus mu scles were examined for myosin isoform composition, fibre type, contra ctility and sarcoplasmic reticulum (SR) Ca2+ release properties either in control rats or in rats treated with thyroid hormone (3,5,3'-triio dothyronine, T-3) for 4 weeks. 2. T-3 treatment had a strong impact on myosin heavy chain (MyHC) and light chain (MyLC) isoform composition in both young and old rats. That is, all single fibres co-expressed ty pe I and IIA (type I/IIA fibres) or type I, IIA and IIX MyHC's (type I /IIAX fibres) after treatment. Slow and fast MyLC isoforms, i.e. MyLC1 s, MyLC1f, MyLC2s, MyLC2f and MyLC3, co-existed in each of the type I/ IIA and I/IIAX fibres in variable proportions. 3. In old rats the maxi mum velocity of unloaded shortening (V-0) was related to MyHC isoform composition: V-0 for type I fibres was less than that for type I/IIA f ibres which nas less than that for type I/IIAX fibres. In young rats, on the other hand, V-0 did not differ between pure type I fibres from controls and those co-expressing type I and type II MyHC isoforms from T-3-treated rats. 4. Contraction and half-relaxation times of the iso metric twitch were significantly longer in old than in young controls. This was paralleled by an age-related decrease in the caffeine thresh old of the SR. Four weeks of T-3 treatment eliminated the age-related differences in both speed of twitch contraction and caffeine threshold s. V-0, on the other hand, was slower in old than in young animals, bo th control and T-3-treated, when cells with a similar MyHC composition were compared. 5. In conclusion, thyroid hormone can substantially re verse at least some of the changes that occur in ageing muscle. Furthe r, the age-related decline in V-0 in soleus fibres from control and hy perthyroid rats suggests that: (1) the identification of beta/slow myo sin isoforms is incomplete; or (2) the molecular characteristics of My HC: differ between young and old age;or (3) MyHC is not the only deter minant of V-0.